1. Field of the Invention
The present invention relates to fluid flow regulators.
2. State of the Art
An ideal fluid flow regulator delivers fluid downstream at a controlled constant flow rate (volume per unit of time) over a wide range of upstream pressure variation. Many known fluid regulators do not achieve the ideal. Some known regulators use a differential pressure drop across a fixed orifice with an active valve correcting flow rates. There are also positive displacement and mass measuring pressure regulators which maintain a fixed pressure drop across an orifice. Differential pressure devices usually require large pressure losses or expensive means for sensing smaller pressure drops, which limit their applications. Positive displacement and mass measuring devices are usually expensive and are often too large for certain applications. Pressure regulators with fixed orifices are also too large for many uses. They are inaccurate in the face of upstream or downstream head pressure variations. Leakage or wear results in worse flow regulation.
Most flow regulators exhibit some degree of positive regulation. Positive regulation is a condition in which flow rate increases with increasing pressure. In many applications, positive regulation is very undesirable. When two fluids are being mixed downstream of the flow regulator, and the pressures on each vary, the proportion of one fluid relative to the other in the mixture can vary to achieve unacceptable results. One area where this occurs is in soft drink dispensing systems. Soft drink dispensers mix syrup and carbonated water to make a soft drink. Slight variations in the percentage of the syrup to water mixture can greatly affect the taste and other quality features of the soft drink.
Current regulators vary .+-.5% in the amount of water and syrup dispensed over pressure variations that occur in these systems. State of the art flow regulators are also quite expensive to make. They rely on precision machined, stainless steel, piston and sleeve assemblies that are spring biased. The precision machining adds greatly to the cost of the regulators. Also spring biased systems, which rely on spring compression over a distance, do not account for the change in the spring constant as a function of compression. Spring constant error introduces non-linearity to the flow regulator.
Chenault, U.S. Pat. No. 2,865,397 (1958), is an example of such a regulator. One of the problems of that regulator, which is a problem common to regulators that use a piston-sleeve arrangement is that some space must be provided between the piston and the sleeve to allow relative motion between them. This causes some fluid flow as leakage through that space. The space, therefore, becomes a variable orifice yielding increased flow rate with respect to increased input pressure. Tolerance problems can also allow the piston to become skewed within the cylinder, which also results in positive or inaccurate regulation. Insofar as the piston in the prior art devices have output ports that are closed by a portion of the sleeve, the ports must be accurately spaced circumferentially about the piston. Unless the openings are evenly spaced, fluid flow will create a forces urging one side of the piston against the sleeve. This creates inaccurate results because one output port will be more or less restricted than other ports. Thus, flow through each of the ports may differ. Therefore, having multiple ports equally spaced around a sleeve results in non-linear displacement versus flow rate as input pressure changes. If closer tolerances are attempted to limit skewing and leaking around the piston, higher friction results, which necessitates higher pressure losses across the reference orifice to compensate for these frictional losses.